This study investigates the urban heating system (UHS) by taking a look at the heat transportation flexibility of the system. We propose the heating system flexibility (HSF) concept to represent UHS’s capability of meeting the heating demand under different operation conditions during the heating season and give out the corresponding evaluation method. Based on the evaluation method, we investigate the impact of heating network enhancement measures upon HSF by taking a real UHS in Beijing as a demo site. We pick network-wise topological change (extra pipe) and booster pump installation as two representative renovation measures. When an extra pipe close to end-user is introduced to the network, the average flexibility increases but the median flexibility drops. The results show that the introduction of the extra pipe does not reduce the hydraulic imbalance among different substations. A booster pump is more suitable for improving local substation HSF, although such a measure is only effective to a portion of the substations. Overall, the concept of HSF has the potential of being used as an important criterion in the design, operation, and control of UHS and other energy systems.